1. Field of the Invention
The present invention relates in general to electric motors and more particularly to the electric motors of an axial gap type which comprises a rotor shaft that is rotatable about its axis, at least one rotor that is fixed to the rotor shaft to rotate therewith and at least one stator that is disposed around the rotor shaft and axially spaced from the rotor.
2. Description of the Related Art
Hitherto, various radial gap electric motors have been proposed and put into practical use particularly in the field of power generators that need higher power density and lower heat generation. Some of them are disclosed in Japanese Laid-open Patent Applications, Tokkaihei 11-341758 and Tokkai 2000-224836.
The motor shown by 11-341758 comprises a cylindrical stator, an outer rotor rotatably disposed around the cylindrical stator and an inner rotor rotatably disposed in the cylindrical stator. While, the motor shown by 2000-224836 comprises a cylindrical stator, an outer rotor rotatably arranged in a diametrically outer side in the cylindrical stator and an inner rotor rotatably arrange in a diametrically inner side in the cylindrical stator.
In both of the motors mentioned hereinabove, by feeding the stator with a compound current, the outer and inner rotors are forced to rotate independently from each other.
In the motor of the latter reference, viz., 2000-224836, it is considerably difficult to provide the inner rotor with a sufficient size due to the inevitably limited space defined in the diametrically inner area of the cylindrical stator, and thus, it is difficult to expect a sufficient torque from such small sized inner rotor. In view of this drawback, electric motors of the type of the former reference, viz., 11-341758 have been widely used in these days.